The specification of UK Patent Application GB 2 187 858 A describes an electrically controllable variable optical power ratio single mode optical fibre branching element constituted by a tandem arrangement of two 4-port 3 dB fused fibre couplers in Mach Zehnder configuration. Such a device is depicted schematically in FIG. 1 in which a first 4-port 3 dB single mode fused fibre coupler 10 with ports 10a, 10b, 10c and 10d, and a coupling region 10e, is optically coupled by means of two lengths 12 and 13 of single mode fibre with a second 4-port 3 dB single mode fused fibre coupler 11 having ports 11a, 11b, 11c and 11d, and a coupling region 11e. The lengths 12 and 13 of single mode optical fibre, which extend respectively between ports 10b and 11a, and between ports 10c and 11d, constitute the two interference arms of the Mach Zehnder configuration. Accordingly, if the optical path lengths of these interference arms 12 and 13 are equal, then all light launched into port 10a of the configuration emerges from port 11c, and similarly all light launched into port 10d emerges from port 11b. (The optical path length of any stretch of waveguide in which light propagates is the product of its physical length with the effective refractive index of light propagating in the guide.) If the two arms are of unequal optical path length, then the light that is launched into port 10a is shared between ports 11b and 11c in a ratio determined by the difference in phase introduced by the difference in optical path length. For any given wavelength, increasing the optical path length difference will cause the proportion of the light reaching port 11c from port 10a to vary according to a raised cosine characteristic. If the power from port 10a that emerges by way of port 11b is absorbed or otherwise disposed of, the optical coupling between port 10a and port 11b can be viewed in terms of the configuration acting as an optical attenuator. By the introduction into one of the interference arms of some form of electrically biased optical path length adjuster 14, the coupling between port 10a and port 11b can now be viewed in terms of the configuration acting as an electrically controllable optical attenuator. In the case of the specific configuration described in GB 2 187 858 A, the adjuster is an electrostrictive adjuster which changes the optical path length by physical stretching of one of the interference arm fibres.
Since the value of attenuation provided by the device is determined by the phase difference introduced by the difference in optical path length of the two interference arms, the value of attenuation provided by the configuration is inevitably wavelength dependent. For some applications this wavelength dependence may be small enough to be acceptable, while for others it may be too large. In respect of a configuration with equal optical path length interference arms under zero bias conditions, FIG. 2 shows calculated variations in attenuation over the (free-space) wavelength range 1530 nm to 1560 nm for different amounts of optical path length imbalance by a localised raising, by heating, of the effective refractive index of a 1 mm portion length in one of the interference arms. This figure shows that over this wavelength range the spectral variation in attenuation is proportionately quite small for attenuation of up to about 5 dB, but for attenuations of about 15 dB or more the spectral variation in attenuation has become much more significant.